Polypropylene is a very desirable material because of its low cost and good physical properties. In many applications, the stiffness of polypropylene must be improved by the addition of a reinforcing filler such as mica. Mica is a suitable reinforcing filler because of its good properties and its low cost which further reduces the cost of a mica-filled polypropylene compound.
A drawback encountered in using a mica-filled polypropylene is the low impact property of the material. By adding mica, the stiffness of the material is improved at the expense of its impact properties. It is known that the impact properties of polypropylene may be improved by blending with an ethylene-propylene-diene monomer (EPDM) rubber. However, this rubber component must be carefully added to the total composition in order not to sacrifice substantially the stiffness or the flexural modulus properties.
In compounding a polypropylene composition containing both a mica filler and an impact modifier rubber component, a melt extrusion and pelletizing operation is normally required. Due to the dissimilar shapes and forms of the raw material involved, i.e., the rubber and the polypropylene are generally in the form of pellets while mica is in powder form, a homogeneous blend of the composition is not possible unless it is compounded in a melt extrusion process where extensive mixing is possible for the polymers in their molten state. A melt extrusion process produces a three component polypropylene composition in rod form. The rods are then chopped into pellets to be used in subsequent injection molding processes to make the end product. This intermediate processing of a mica filled, rubber modified polypropylene composition contributes to a substantial portion of the total cost of the blended material. All commercially available mica-filled, impact modified polypropylene compositions are prepared by the melt extrusion and pelletizing method and therefore making it a high cost material. The high cost of these commercially available materials making polypropylene less competitive with other low cost thermoplastic materials such as polyvinyl chloride and polyethylene.
It is therefore an object of the present invention to provide a mica-filled, impact modified polypropylene composition which can be compounded in a dry blending operation to be used in an injection molding process directly without the melt extrusion and pelletizing operations.
It is a further object of the present invention to provide a mica-filled, impact modified polypropylene composition which is dry blendable and has superior stiffness and impact properties.